1. Field of the Invention
The present invention relates to processes and apparatuses for producing cast-coated papers, particularly to processes for producing cast-coated papers for ink jet recording, which can effectively prevent curling in a wide range of environments as well as curling during printing/recording.
2. Description of Related Art
Cast-coated papers are characterized by higher sheet gloss and better surface quality as compared with ordinary coated papers because the surface feature of the mirror surface of a casting drum is transferred onto the paper. Conventionally, cast-coated papers have been widely used for various purposes including commercial printing such as the covers of magazines and posters; bags such as high-quality shopping bags and fancy packing boxes; release papers for adhesive labels; ink jet recording for ink jet printing and the like. It has become especially possible recently to achieve very high quality full-color printing close to silver salt photographic printing using cast-coated papers for ink jet recording by improving the suitability of cast-coated papers for use in ink jet recording such as ink absorbency and image reproducibility.
Generally, processes for producing cast-coated papers are roughly classified into the following categories: the direct process involving applying a coating color onto the surface of a base paper and then immediately pressing the coated layer in the wet state against a casting drum with forming rolls; the gel cast coating process involving applying a coating color, then passing the coated layer through a gelling bath to convert it into a malleable gel state with plasticity and then pressing it against a casting drum; the re-wet cast coating process involving the application of a coating color to the paper and once drying, to give the paper a dry coated surface and then rewetting/plasticizing the coated surface with water or an appropriate rewetting solution and pressing it against a casting drum, etc.
All of the above-described processes provide a glazed finish by applying a coating color containing a pigment and an adhesive onto a base paper and pressing the coated layer side of the paper plasticized with water, against the surface of a heated casting drum having a mirror surface with forming rolls (press rolls), and drying and releasing the paper. In general, cast-coated papers show a great difference in the coating amounts used on the two sides of the paper and, therefore tend to be more liable to curling than ordinary coated papers when exposed to atmospheric changes and fluctuations in humidity. This is so especially with cast-coated papers provided with ink jet-printability as a coated layer which have a strong tendency to undergo curling when exposed to changes in humidity; presumably because of their good affinity to water.
When storing paper that has been cut into sheets for use in offset printing, it is necessary to keep the paper flat and pressed because offset printing paper which has been cut into sheets and stacked can absorb moisture from the surrounding atmosphere if the moisture content of the paper is low, and therefore, has a greater tendency to curl and warp when printing is performed on the cast-coated surface or the opposite surface.
Ink jet recording is a recording process of depositing small ink particles that are projected by various mechanisms onto a recording paper to form dots having the advantage of being readily adaptable to full-color printing and performing high-speed printing. However, the ink jet recording process has the disadvantage that a significant level of curling occurs after printing, depending on the kind of paper used, because of swelling (with the aqueous inks used in printing) and shrinking (by drying) of the fibers of the base paper. In an attempt to reduce this curling, the non-cast-coated surface of the printing paper was acted upon by water vapor to give 1.0 g of moisture per m2 of paper (see JPA HEI 9-11607, pp. 3-9), but the product significantly curled in the direction of the non cast-coated surface. Moreover, the treatment with water vapor was found to be insufficient with regard to reducing waving and curling of the printing paper when exposed to environmental change.